How fast is a fly inside a moving car?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

15 July 2007

Arthur says that he is “one of those people who have despised figures” his whole life… “to the point whereby it took me almost three years to memorise my personal cell phone number.” Nevertheless, he has been wondering about flies in cars.

He asks; “If a person in a car travelling at 100 km/h is also moving at 100 km/h: how fast is a fly travelling if it flies from the back seat to the front seat? Secondly how fast is the fly travelling if it flies from the back seat to the front?”

Before answering that, we must first ask how fast a fly flies in open air. Now I have seen these insects whiz across my five-metre room in about two seconds. That works out to 2.5 metres per second, or 9km/h. But let’s call it 10km/h to simplify matters.

If you are standing on the side of the road while the car passes by at 100km/h and the fly flies (inside the vehicle) from the back to the front, you would record the insect’s speed as 110km/h. For the reverse journey, the speed would be 90km/h.

But another interesting question arises. Suppose the fly was floating in the air motionlessly near the front of the car and the car was also stationary. When the car begins to move, will the fly be “left behind” (eventually being hit by the rear wind-screen)?

The answer is no. The reason is that the fly is being supported by the air in the car. When the car starts to move, it takes the air along with it – that is why occupants in the car don’t feel wind blowing into their eyes (unless they are seated next to an opened window where the current flows from outside).

The fly therefore moves with the air…which in turn moves with the car.

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Cyrus Njoroge says he has an idea that would create a perpetual motion machine. He says: “One can install a windmill to pump underground water to a high tank. This is then connected to a drainage pipe in which a turbine is installed to generate electricity. Afterwards, the runoff is left to flow back to the underground reservoir thus ensuring there's no wastage. Since it will be using wind power/energy, which is renewable and underground water which will be replenished back, I think this is energy that will last us to perpetuity.

Well Cyrus, the machine would work. But, instead of pumping water, why not simply connect the generator directly to the windmill? That way, we wouldn’t have to worry about the friction in the water pump and the turbine.

The problem with Njoroge’s system is that it has many unnecessary moving parts – water in pipes, the pump, the turbine. All these are “loopholes” for the tapped wind energy to escape through. For that reason, it is better to put the dynamo right next to the windmill…and such systems are already in operation in many countries, including Kenya.